1) Field of the Invention
The present invention relates to a method and an apparatus for etching a compound semiconductor in the production of semiconductor devices, and particularly to a method for etching a compound semiconductor under precise control of etching depth.
2) Related Art
With recent requirements for making semiconductor devices it has been proposed to etch semiconductors precisely in the order of thickness of a few monoatomic layers. For example, according to a method disclosed in JP-A 3-110844, a step of adsorbing chlorine as a reactive gas on the surface of GaAs as one of compound semiconductors, and a step of energizing the surface by irradiation with charged particle beams, thereby occasioning reactions and desorbing the reaction products from the surface are repeated alternately, where the etching depth can be controlled by selecting number of repetitions of these two etching steps and precise etching in the order of thickness of a few monoatomic layers can be obtained.
Other known methods for desorbing the reaction products include a method for highly evacuating an etching chamber (JP-A 2-213130) and a method for heating a semiconductor substrate to a high temperature (JP-A 2-220436). Another example of controlling an etching depth by repetitions of etching steps is a method for repeating a step of heating silicon as one of semiconductors in an oxidizing atmosphere, thereby forming an oxide film on the surface and a step of removing the oxide film by etching with a hydrofluoric acid solution (JP-A 61-183923).
In the foregoing methods disclosed in JP-A 3-110844, JP-A 2-213130 and JP-A 2-220436, it is utilized to interrupt adsorption of the reactive gas on the semiconductor surface in a thickness level of a monoatomic layer to make constant the amount of the semiconductor to be removed by one run of the etching steps. However, the amount of the reactive gas to be adsorbed can be no more than that for the thickness of a monoatomic layer in that case and thus the etching depth obtained by one run of the etching steps will be less than the thickness of a monomolecular layer. Monomolecular layer corresponds, in case of gallium arsenide, to the total thickness of a monolayer of gallium and a monolayer of arsenic. For example, when chlorine (Cl) is adsorbed onto the gallium arsenide (100) surface, the arsenic chloride having a higher vapor pressure will be desorbed immediately after its formation, whereas the gallium chloride cannot be desorbed due to its relatively low vapor pressure. As a result, a monolayer of chlorine atoms is adsorbed on the gallium surface. Then, the chlorine is subjected to reaction with gallium, where the reaction product is GaCl.sub.3. That is, three chorine atoms are required for one gallium atom, and only one-third of the monoatomic layer of gallium can be removed by one run of the etching steps. Thus, only two-thirds of the monoatomic layer of gallium arsenide can be removed by one run of the etching steps and the etching depth by one run of the etching steps will be less than the thickness of a monomolecular layer. In the ordinary process for producing semiconductors, at least centi (100)-molecular layer must be removed in most cases, and thus there is such a problem as a necessity for carrying out too many runs of the etching steps.
In the method disclosed in the foregoing JP-A 61-183923 the amount of semiconductor to be removed by one run of the etching steps can be made more than the monoatomic layer thickness (monomolecular layer thickness in case of compound semiconductor) by increasing the thickness of oxide film layer. However, the oxygen atom is considerably smaller than any of semiconductor constituent atoms, and thus the oxidation reaction of semiconductor proceeds by faster internal diffusion taking place along the interstitical sites. That is, it is difficult to control the thickness of oxide film and thus it is difficult continuously to obtain an oxide film having a constant thickness. Thus, there is such a problem that the amount of semiconductor to be removed by one run of the etching steps is liable to fluctuate.